A typical mixture in emulsion polymerization consists of water, monomer(s), an initiator (usually water-soluble) and an emulsifier. The role of the emulsifier is critical and multifaceted. Initially it serves to form and stabilize an emulsion of the starting materials. Later, some of the initial micelles and/or emulsified droplets serve as the locus for polymer particle nucleation. Lastly, the emulsifier serves in the stabilization of the final latex. Most commonly, the emulsifier is a low molar mass surfactant, however, variations include systems such as “emulsifier-free” recipes wherein the surfactants are created in-situ. This is accomplished by either copolymerization of a hydrophilic co-monomer or by oligomerization of the hydrophobic monomer by a hydrophilic, generally an ionic initiator fragment. Additionally, copolymerizable surfactants have been used as well and represent a middle ground between classical and emulsifer-free systems.
As an alternative to the previously discussed systems are polymeric surfactants. Many different molecular architectures are possible with polymeric surfactants such as amphiphilic block and graft copolymers which contain hydrophobic and hydrophilic segments, and “polysoaps” which consist of polymerized reactive surfactants. Polysoaps in many ways are similar to polyelectrolytes in that both are charged polymeric species. The primary difference between them is that the reduced specific viscosity of polysoaps in aqueous solution is far lower than that of normal polyelectrolytes of comparable molecular weight. This critical difference is due to the compact molecular structure of polysoaps attributed to intramolecular micelle formation. These micelles have the ability to solublize hydrocarbons but unlike conventional surfactants, no critical micelle concentration is required. Typical polyelectrolytes such as poly(4-vinylpyridine) can transition to a polysoap by alkylation of a portion of the pyridine groups with dodecyl bromide. The dodecyl groups then undergo intramolecular aggregation due to hydrophobic attraction resulting in micelle formation and a sharp drop in intrinsic viscosity.
The use of block copolymers as polymeric surfactants in emulsion and dispersion polymerization has a long history. The practical challenge has been to develop simple, economical synthetic techniques to “tailor-make” the precise molecular characteristics required of block copolymer surfactants. This goal is now being realized by the remarkable progress made in recent years by controlled free radical polymerization (CFRP) such as reversible addition-fragmentation chain transfer (RAFT). With CFRP, block copolymers of defined structure, molecular weight and polydispersity are becoming commonplace. Many block copolymers formed in this way have already been demonstrated to function as polymeric surfactants for emulsion polymerization.
While many examples of emulsion polymerizations carried out with both polysoaps and block copolymeric surfactants exist, we have found only one example where a polyelectroyte homopolymer could be directly used in an aqueous solution to prepare a self-stablizing block copolymer latex using one or more relatively hydrophobic free-radically polymerizable monomers in the presence of an initiator without the aid of additional surfactant. This example doesn't use a RAFT system but rather a nitroxide-mediated (NMP) controlled free radical technique based on a water soluble alkoxyamine initiator. The NMP initiator is first used to prepare an aloxyamine-terminated poly(arcylic acid) macroinitiator with a defined molecular weight and polydispersity by solution free radical polymerization at 120° C. The poly(acrylic acid) macroinitiator is then dissolved at room temperature in aqueous sodium hydroxide solution to obtain the polyelectrolyte macroinitiator; alkoxyamine-terminated poly(sodium acrylate). Addition of either styrene or butyl acrylate to the aqueous solution yields an unstable biphasic system. However, heating this stirred mixture to 120° C. for 8 hrs under nitrogen produced stable latexes with over 90 percent monomer conversion. It is also important to realize that the latexes produced by this technique are anionically stabilized by surface negative charges.
The primary disadvantages of this system are the synthesis of the required alkoxyamine and the temperature of polymerization being above the boiling point of water.